Re: Line Voltage Has Offset

On Jan 17, 4:43 pm, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Fri, 18 Jan 2008 10:23:29 +1100, "Phil Allison"



<philalli...@xxxxxxxxxx> wrote:

"D from BC"
I did an oscilloscope diff. test on a wall outlet.
The sine was offset??

|<----scope--->|
Outlet ---N------x10 Probe tip 1------Ch 1
|
Outlet----G-------------------------- Earth
|
Outlet----L-----x10 Probe tip 2-------Ch 2 Inv

I confirmed with a DMM on DC setting.
Approx. 10VDC with respect to L.

Just a bit curious how the line gets offset.

** Totally WRONG test procedure.

To measure a small DC offset in the presence of a large AC voltage with a
DMM - you must first remove most of the AC voltage with a low pass filter.

Try using a 1 megohm resistor in series with a 1 uF film cap wired across
the AC supply - then measure the DC component across the cap with your
DMM. Allow for the effect of the resistor if you like, it reduces the DC
reading by about 10%.

Bet the new result is way under 1 volt.

........ Phil

Bummer...
I think that was probably a classic DMM gotcha. :(
Too much faith in my DMM. :P
I just fired myself... :P

Oh!
Just got rehired. :)

I did the RC filter circuit as suggested ..
Line offset voltage = 0.0001VDC (zilch..)
That's more like it.. :)

I dunno wtf I did wrong with the scope set up..
I repeated the set up and the scope shows no offset.
Must have goofed a vertical setting.
Double fooled..

.... or there was a DC offset and now there isn't.
Was the neighbor doing some sort of "science experiment" involving
passing large DC currents into a "ground rod"? :) :)

Do you live in the SanFransisco area near a BART station?




Embarrassed :(

D from BC
British Columbia
Canada.

.

Relevant Pages

  • Re: Line Voltage Has Offset
    ... I confirmed with a DMM on DC setting. ... Just a bit curious how the line gets offset. ... DMM - you must first remove most of the AC voltage with a low pass ... I dunno wtf I did wrong with the scope set up.. ...
    (sci.electronics.design)
  • Re: Line Voltage Has Offset
    ... Just a bit curious how the line gets offset. ... To measure a small DC offset in the presence of a large AC voltage with a DMM - you must first remove most of the AC voltage with a low pass filter. ... I dunno wtf I did wrong with the scope set up.. ...
    (sci.electronics.design)
  • Re: Line Voltage Has Offset
    ... Just a bit curious how the line gets offset. ... To measure a small DC offset in the presence of a large AC voltage with a DMM - you must first remove most of the AC voltage with a low pass filter. ... I dunno wtf I did wrong with the scope set up.. ...
    (sci.electronics.design)
  • Re: Line Voltage Has Offset
    ... Just a bit curious how the line gets offset. ... To measure a small DC offset in the presence of a large AC voltage with a DMM - you must first remove most of the AC voltage with a low pass filter. ... I dunno wtf I did wrong with the scope set up.. ...
    (sci.electronics.design)
  • Re: Line Voltage Has Offset
    ... Just a bit curious how the line gets offset. ... DMM - you must first remove most of the AC voltage with a low pass filter. ... I dunno wtf I did wrong with the scope set up.. ...
    (sci.electronics.design)
Quantcast